This invention relates generally to toner image creation and more particularly to developability control which enables a wider usable A.sub.t (i.e. a toner material's effectiveness in charging with a given carrier) range.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoreceptor. The photoreceptor comprises a charge retentive surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
A common type of developer comprises carrier granules having toner particles adhering triboelectrically thereto. The two-component mixture is brought into contact with the photoconductive surface, where the toner particles are attracted from the carrier granules to the latent image. This forms a toner powder image on the photoconductive surface which is subsequently transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
Most xerographic engines employ either a toner concentration sensor or measure the reflectance from a constant-potential solid area test patch to implement developability control. These approaches allow use of only a small fraction of the total Toner Concentration -Tribo (TC-Tribo) latitude space which is of special concern with color developer materials.
The challenge is to find a control strategy which, in the presence of sensor noise and drift, enables use of at least 3/4 of the available A.sub.t latitude.
In conventional two-component xerographic development, the ability of a toner material to charge with a given carrier material is quantified as follows: EQU A.sub.t =Tribo * (TC+C.sub.0)
where Tribo is the average charge to mass ratio of toner, TC is the toner concentration in percent by weight, and C.sub.0 is a constant. A.sub.t is a critical specification parameter for toner and developer; it tends to vary from batch to batch, with developer age, and with operating relative humidity. The variation with humidity is a special problem with many color toners, since this variation tends to be much larger than with comparable black toners. Considerable effort has been expended in recent years to formulate developer materials with improved A.sub.t stability, but variations of .+-.70% with respect to the nominal value remain common at environmental extremes.
The ability of the xerographic engine to tolerate large A.sub.t variations and still deliver acceptable print quality can be shown graphically via a TC-Tribo latitude plot, a typical example of which is shown in FIG. 1. This plot shows the locus of print quality specification boundaries at fixed (optimized) values of development and cleaning potential. The interior of the closed zone or area in FIG. 1 represents a region of acceptable print quality. Lines of constant A.sub.t cross the zone diagonally; those which intersect the closed zone represent allowable operating values in principle. For the example, as shown in FIG. 1, the range of potentially allowable A.sub.t values is 125 units, from 25 to 150.
In practice, differences between the toner consumption rate and the dispense rate will always produce fluctuations in toner concentration, even if A.sub.t remains constant. In any high quality xerographic engine, a developer control system must be provided to minimize those fluctuations in TC. As A.sub.t changes from its nominal value, each type of control system will follow a distinctive path through the latitude space. The net result is that each toner control approach can be characterized by a nominal control line and a control band (due to varying consumption, sensor noise and drift) in TC-Tribo latitude space. The overlap between this control band and the print quality acceptance zone defines the allowable range of A.sub.t values for a given control strategy. This range is always less than that shown in FIG. 1.
FIG. 2 shows a typical control line and control band (shaded area) for a toner control strategy based on the use of a toner concentration sensor mounted in the developer housing. The allowable A.sub.t range is only about 20 units; this is only 1/6 of the available latitude.
FIG. 3 shows a typical control line and control band for a toner control strategy based on the measurement of reflectance from a fixed-potential solid area test patch. The allowable A.sub.t range is about 40 units, or about 1/3 of the available latitude. This range would be adequate for many black developers, but it is too small for many color developers when exposed to humidity changes.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the patentability thereof, these references, together with the detailed description to follow hereinafter, may provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 5,210,572 granted to McDonald et al on May 11, 1993 and assigned to the same assignee as the instant invention discloses a toner dispenser control strategy wherein Infra-Red Densitometer (IRD) readings of a developed toner patch in a tri-level imaging apparatus are compared to a target value stored in Non-Volatile Memory (NVM) and are also compared to the previous IRD reading. Toner dispensing decisions (i.e. addition or withholding) are based on both comparisons. In this manner, not only are IRD readings examined as to how far the reading is from the target value but they are examined as to current trend (i.e. whether the reading is moving away from or toward the target.
If the IRD reading indicates that the toner concentration is low but is heading toward the target then the amount of added toner is somewhat reduced. If the IRD reading indicates that the toner concentration is low and is heading away from target (getting lower) then some extra toner is dispensed.
U.S. Pat. No. 5,227,270 granted to Scheuer et al on Jul. 13, 1993 discloses a single pass tri-level imaging apparatus, wherein a pair of Electrostatic Voltmeters (ESV) are utilized to monitor various control patch voltages to allow for feedback control of Infra-Red Densitometer (IRD) readings.
The ESV readings are used to adjust the IRD readings of each toner patch. For the black toner patch, readings of an ESV positioned between two developer housing structures are used to monitor the patch voltage. If the voltage is above target (high development field) the IRD reading is increased by an amount proportional to the voltage error. For the color toner patch, readings using an ESV positioned upstream of the developer housing structures and the dark decay projection to the color housing are used to make a similar correction to the color toner patch IRD readings (but opposite in sign because, for color, a lower voltage results in a higher development field).
Another method of controlling toner dispense rate, useful in electronic printers utilizes the number of character print signals applied to print head. The print signals may be in character code and a statistical average take-out rate used to estimate toner depletion, or the signals may be picture elements (pixel) signals. See for example U.S. Pat. Nos. 3,529,546 and 4,413,264.
U.S. Pat. No. 4,847,659 describes an electrostatographic machine which replenishes toner in a developer mix in response to a toner depletion signal which represents the toner usage rate. The toner depletion signal is determined from the number of character print signals applied to a print head, or in other words, the number of pixels to be toned. The depletion signal is used in conjunction with a second signal, which represents a proportional toning contrast, such that the constant of proportionality between the toner depletion signal and a toner replenishment signal is adjusted according to the second signal.
U.S. Pat. No. 5,204,699 granted to Birnbaum et al on Apr. 20, 1993 relates to an apparatus for estimating the mass of toner particles developed on a latent electrostatic image. The apparatus includes converting means for approximating the mass of the toner required to develop an output pixel as a function of the image intensity signal which is used to control the exposure of the output pixel. Also included is summing means, responsive to the toner mass signal, which determines the sum of the approximated toner mass over a plurality of output pixels, thereby producing a sum signal representing the estimated toner mass developed on the output pixels.
U.S. Pat. No. 5,204,698 granted to LeSueur et al relates to a laser printer in which a latent image is generated on a circulating imaging member in accordance with digital image signals and subsequently developed with toner, the number of pixels to be toned is used as an indication of the rate at which toner is being depleted from the developer mixture. The device for dispensing fresh toner to the developer mixture is operated in pre-established relationship between the pixel count and the length of time for which the dispensing device is in operation. If the efficiency of the dispensing device falls, the pre-established relationship is adjusted so that the toner density in the developed images remains constant. If a predetermined level of adjustment is reached, it is taken as an indication that the supply of toner in the printer is low, and should be replenished.
U.S. Pat. No. 5,202,769 granted to Tadaomi Suzuki on Apr. 13, 1993 discloses image output apparatus including a circuit for counting the number of pixels of various color and gradation densities contained in the image data, a circuit for estimating, based on the counted number, the amount of toner that will be consumed during development of the image data; and means for controlling, based on the estimated amount, the actual amount of toner supplied for developing the image.